We did partially reproduce that sound using a Rhodes run through a pair of Live's Resonators (I'll have to find the files), but occasionally we'll hear something like a distant car on the freeway that sounds an awful lot like "the sound".

Another really fun one was what the Russian Navy referred to as "Quackers", which have since been determined to be an abnormal call of Antarctic minke whales. There are some cool and weird sounds out there!